Poly(isobutylene-co-isoprene) or IIR, is a synthetic elastomer commonly known as butyl rubber (or Butyl polymer) which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (usually not more than 2.5 mol %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Halogenation of butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company © 1987), particularly pp. 297-300.
The development of halogenated butyl rubber (halobutyl) has greatly extended the usefulness of butyl by providing much higher curing rates and enabling co-vulcanization with general purpose rubbers such as natural rubber and styrene-butadiene rubber (SBR). Butyl rubber and halobutyl rubber are high value polymers, as their unique combination of properties (excellent impermeability, good flex, good weatherability, co-vulcanization with high unsaturation rubbers, in the case of halobutyl) make them preferred materials for various applications, such as their use in making tire inner tubes and tire inner liners.
The presence of allylic halide functionalities allows for nucleophilic alkylation reactions. It has been recently shown that treatment of brominated butyl rubber (BIIR) with nitrogen and/or phosphorus based nucleophiles, in the solid state, leads to the generation of IIR-based ionomers with interesting physical and chemical properties (see: Parent, J. S.; Liskova, A.; Whitney, R. A; Resendes, R. Journal of Polymer Science, Part A: Polymer Chemistry 43, 5671-5679, 2005; Parent, J. S.; Liskova, A.; Resendes, R. Polymer 45, 8091-8096, 2004; Parent, J. S.; Penciu, A.; Guillen-Castellanos, S. A.; Liskova, A.; Whitney, R. A. Macromolecules 37, 7477-7483, 2004). The ionomer functionality is generated from the reaction of a nitrogen or phosphorus based nucleophile and the allylic halide sites in the halogenated butyl rubber to produce an ammonium or phosphonium ionic group respectively.
Like other rubbers, for most applications, butyl rubber must be compounded and vulcanized (chemically cross-linked) to yield useful, durable end use products. Grades of butyl rubber have been developed to meet specific processing and property needs, and a range of molecular weights, unsaturation, and cure rates. Both the end use attributes and the processing equipment are important in determining the right grade of butyl rubber to use in a specific application.
Peroxide curable butyl rubber compounds offer several advantages over conventional sulfur-curing systems. Typically, these compounds display extremely fast cure rates and the resulting cured articles tend to possess excellent heat resistance. In addition, peroxide-curable formulations are considered to be “clean” in that they do not contain any extractable inorganic impurities (e.g., sulfur). The clean rubber articles can therefore be used, for example, in condenser caps, biomedical devices, pharmaceutical devices (stoppers in medicine-containing vials, plungers in syringes) and possibly in seals for fuel cells.
One approach for obtaining a peroxide-curable butyl-based formulation lies in the use of conventional butyl rubber in conjunction with a vinyl aromatic compound like divinylbenzene (DVB) and an organic peroxide (see Japanese Publication No. 06-107738). In place of DVB, an electron-withdrawing group-containing polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,N′-m-phenylene dimaleimide) can also be used (Japanese Publication No. 06-172547).
A commercially available terpolymer based on isobutylene (IB), isoprene (IP) and DVB, XL-10000, is curable with peroxides alone. This material, however, possesses significant levels of DVB. In addition, since the DVB is incorporated during the polymerization process a significant amount of crosslinking occurs during manufacturing. The resulting high Mooney (60-75 MU, ML1+8@125° C.) and presence of gel particles make this material difficult to process.
Canadian Patent No. 2,418,884 and Canadian Patent Application No. 2,458,741 describe the preparation of butyl-based, peroxide-curable compounds which have high multiolefin content. Specifically, CA 2,418,884 describes the continuous preparation of IIR with isoprene levels ranging from 3 to 8 mol %. The elevated level of isoprene in the polymer backbone renders these compounds peroxide curable. Halogenation of this high multiolefin butyl rubber consumes some of this unsaturation and produces a reactive allylic halide functionality within the elastomer. With these elevated levels of isoprene, it is possible to generate BIIR analogues which contain allylic bromide functionalities ranging from 3 to 8 mol %, often with residual double bonds in the polymer backbone. Nucleophilic substitution reactions as described above can be used to create ionomeric moieties from these allylic halide sites, with the residual unsaturation being sufficient to permit peroxide curing. Peroxide curable butyl rubber ionomer compositions with elevated levels of isoprene are described PCT Publication Nos. WO2007/022618 and WO2007/022619.
One of the key failures for elastomeric compounds in dynamic applications is crack growth. Butyl ionomers possess ionic functionalities in addition to unsaturation. When these compounds are cured, there are both reversible ionic crosslinks as well as irreversible chemical crosslinks. The reversible ionic crosslinks allow the butyl ionomers to behave in a “self-healing” manner wherein the polymer chains are able, to a certain extent, flow or be mobilized.
It would therefore be desirable to have butyl rubber ionomer compositions which exhibit better dynamic properties, and physical properties.